Police traffic surveillance devices emit an electromagnetic signal in the radio frequency (RF) band or light band (i.e., infrared, visible, and ultraviolet light) that reflect off of approaching or departing vehicles to determine their speed. In particular, a change in frequency (Doppler shift) or a change in time of travel for return signal pulses is sensed for calculating vehicle speed. The following RF (radar) frequency bands are used: X-band (10.525 GHz±25 MHz); K-band (24.150 GHz±100 MHz); and Ka-band (34.700 GHz±1300 MHz). Furthermore, laser wavelength of 904 nm with 33 MHz bandwidth is also used.
Police radar and laser detectors (“detectors”) are used by drivers of vehicles to detect radiant electromagnetic signals characteristic of police traffic surveillance devices. These detectors are generally a detachable device clipped to a visor or dash of the vehicle for unimpeded sensing of the signals, and for providing a conveniently positioned display and one or more controls to the driver.
Various circuit architectures and techniques have been utilized for the detection of police radar and laser signals, as disclosed in various patents owned by the present assignee and others, and used in products of the present assignee and others. Among these is the circuit architecture shown in U.S. Pat. Nos. 5,900,832 and 5,856,801, which show a radar detector having an antenna coupled to one or more low noise amplifiers (LNA's), providing gain of received signal in a specific police radar band(K, Ka or X). The low noise amplifiers are coupled to a common mixer. A local oscillator signal operating at a K, Ka or X band frequency downmixes the received K, Ka or X band signal to an intermediate frequency for detection. U.S. Pat. No. 5,068,663 discloses a LNA preamplifier only on the X band, coupled to a first mixer, with a passive K/Ka path coupled to a second mixer.
Recently there has been interest in reducing radio emission by detectors. Detectors typically produce radiation as a consequence of the use of a local oscillator in the detector for downmixing received radar signals. Emissions from the local oscillator typically escape the detector by passing through the mixer and into the antenna, and then out the antenna to the surrounding space. Emissions may also propagate directly from the local oscillator circuit through the detector's case into the surrounding space. Even if the case is of metal or other conductive material, emissions may escape through cracks or gaps in the case.
While radar detectors are mobile products and thus not FCC regulated, there have been issues of interference between radar detectors (one detector creating a false signal on another), as well as complaints of VSAT (very small aperture terminal) ground terminal operators about possible interference originating from radar detectors.
While the patents and products heretofore known have disclosed various detector circuit architectures, and those architectures may have exhibited increasing detection performance, there remains significant outward emission of radiation from such detectors, primarily owing to feedthrough from the local oscillator to the antenna and leakage through the case. As to the latter issue, while detectors have been made with metal or conductive cases to form a Faraday cage around the detector circuits, gaps in the case undermine the Faraday cage effect and can permit radiation leakage. The only known approach for sealing gaps in a detector case has been to solder those gaps closed, as implemented by BEL on some of its detectors. This approach may improve the Faraday effect of the case, but at the potential expense of impeding maintenance or upgrading of the detector if either requires opening the case.
Therefore, a significant need exists for a police radar/laser detector that has a reduced radiation signature as compared to those detectors presently available on the market. It is important emission reductions be achieved without compromising detector functions or serviceability.